tools: mkfwumdata: add logic to append vendor data to the FWU metadata

The version 2 of the FWU metadata allows for appending opaque vendor
specific data to the metadata structure. Add support for appending
this data to the metadata. The vendor specific data needs to be
provided through a file, passed through a command-line parameter. Make
corresponding changes to the tool's manpage.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>